U.S. patent application number 12/633703 was filed with the patent office on 2010-04-08 for thin foil for use in packaging integrated circuits.
This patent application is currently assigned to NATIONAL SEMICONDUCTOR CORPORATION. Invention is credited to Jaime A. BAYAN, Ken PHAM, Anindya PODDAR, Nghia Thuc TU, Will K. WONG.
Application Number | 20100084748 12/633703 |
Document ID | / |
Family ID | 44146106 |
Filed Date | 2010-04-08 |
United States Patent
Application |
20100084748 |
Kind Code |
A1 |
PODDAR; Anindya ; et
al. |
April 8, 2010 |
THIN FOIL FOR USE IN PACKAGING INTEGRATED CIRCUITS
Abstract
Methods for minimizing warpage of a welded foil carrier
structure used in the packaging of integrated circuits are
described. Portions of a metallic foil are ultrasonically welded to
a carrier to form a foil carrier structure. The ultrasonic welding
helps define a panel in the metallic foil that is suitable for
packaging integrated circuits. Warpage of the thin foil can be
limited in various ways. By way of example, an intermittent welding
pattern that extends along the edges of the panel may be formed.
Slots may be cut to define sections in the foil carrier structure.
Materials for the metallic foil and the carrier may be selected to
have similar coefficients of thermal expansion. An appropriate
thickness for the metallic foil and the carrier may be selected,
such that the warpage of the welded foil carrier structure is
limited when the foil carrier structure is subjected to large
increases in temperature. Foil carrier structures for use in the
above methods are also described.
Inventors: |
PODDAR; Anindya; (Sunnyvale,
CA) ; BAYAN; Jaime A.; (San Francisco, CA) ;
TU; Nghia Thuc; (San Jose, CA) ; WONG; Will K.;
(Belmont, CA) ; PHAM; Ken; (San Jose, CA) |
Correspondence
Address: |
Beyer Law Group LLP/ NSC
P.O.Box 1687
Cupertino
CA
95015-1687
US
|
Assignee: |
NATIONAL SEMICONDUCTOR
CORPORATION
Santa Clara
CA
|
Family ID: |
44146106 |
Appl. No.: |
12/633703 |
Filed: |
December 8, 2009 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
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12133335 |
Jun 4, 2008 |
|
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12633703 |
|
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Current U.S.
Class: |
257/666 ;
228/111.5; 257/E21.499; 257/E23.01; 428/172; 428/457; 438/112 |
Current CPC
Class: |
H01L 21/4832 20130101;
H01L 2924/01046 20130101; H05K 3/328 20130101; H01L 2924/01029
20130101; H01L 2223/5442 20130101; H01L 2224/97 20130101; H01L
2924/181 20130101; H01L 2924/01013 20130101; H05K 3/025 20130101;
H01L 2924/014 20130101; H01L 2924/01033 20130101; H05K 2203/0152
20130101; H01L 2924/3511 20130101; H01L 2924/01082 20130101; H01L
23/49582 20130101; H01L 2924/00014 20130101; H01L 2221/68345
20130101; H01L 2924/01006 20130101; H01L 21/6835 20130101; Y10T
428/24612 20150115; H01L 24/85 20130101; H01L 24/48 20130101; H01L
2224/85 20130101; H01L 2223/54473 20130101; H01L 24/97 20130101;
H01L 23/544 20130101; H01L 21/568 20130101; Y10T 428/31678
20150401; H01L 2924/00014 20130101; H01L 2924/181 20130101; H01L
2924/00014 20130101; H01L 2924/14 20130101; H01L 2223/54426
20130101; H01L 2224/48091 20130101; H01L 23/3107 20130101; H01L
2224/48091 20130101; H01L 2224/97 20130101; H01L 2224/45015
20130101; H01L 2224/85 20130101; H01L 2224/45099 20130101; H01L
2924/00014 20130101; H01L 2924/207 20130101; H01L 2924/00012
20130101 |
Class at
Publication: |
257/666 ;
428/457; 428/172; 228/111.5; 438/112; 257/E21.499; 257/E23.01 |
International
Class: |
H01L 23/48 20060101
H01L023/48; B32B 15/04 20060101 B32B015/04; B32B 3/00 20060101
B32B003/00; B23K 1/06 20060101 B23K001/06; H01L 21/50 20060101
H01L021/50 |
Claims
1. A method for packaging integrated circuits, comprising:
providing a carrier; providing a metallic foil; and ultrasonically
welding selected portions of the metallic foil to the carrier to
form a foil carrier structure, the ultrasonic welding helping to
define a panel in the metallic foil that is suitable for use in
packaging integrated circuits.
2. The method of claim 1, wherein the ultrasonic welding forms an
intermittent welding pattern that extends along edges of the panel,
the intermittent welding pattern including ultrasonically bonded
portions of the metallic foil interspersed among unbonded portions
of the metallic foil, the unbonded portions being portions of the
metallic foil that have not been ultrasonically welded to the
carrier.
3. The method of claim 2, further comprising: attaching a
multiplicity of dice to the metallic foil; encapsulating the
multiplicity of dice and at least a portion of the metallic foil
with a molding material to form a molded foil carrier structure;
removing the carrier from the molded foil carrier structure to form
a molded foil structure; patterning the exposed foil of the molded
foil structure using photolithographic techniques; etching the
metallic foil after the carrier has been removed to define a
multiplicity of device areas in the metallic foil, each device area
supporting at least one of the multiplicity of dice and having a
multiplicity of electrical contacts, wherein the etching exposes
portions of the molding material; and after the etching step,
singulating the molded foil structure to form a multiplicity of
packaged integrated circuit devices.
4. The method of claim 2, wherein at least a subset of the
ultrasonically bonded portions are arranged linearly and have a
pitch of approximately between 10 and 20 mm.
5. The method of claim 2, wherein the intermittent welding pattern
is arranged into at least four lines of bonded portions, each line
of bonded portions including at least two bonded portions that are
linearly arranged and separated by unbonded portions, the four
lines of bonded portions defining four sides of a rectangular panel
in the metallic foil.
6. The method of claim 2, wherein: the carrier is made of aluminum;
the metallic foil is made of copper; and the thickness of the
metallic foil is between approximately 8 and 35 microns and the
thickness of the carrier is between approximately 7 and 25
mils.
7. The method of claim 2, further comprising: unwinding the carrier
from a carrier coil; unwinding the metallic foil from a foil coil,
wherein the ultrasonic bonding is performed while the metallic foil
and the carrier are in motion and being unwound from the foil coil
and the carrier coil, respectively; before the ultrasonic welding,
conveying portions of the metallic foil and the carrier past a
first set of one or more cleaning stations; at the first set of
cleaning stations, applying cleaning solution to clean the metallic
foil and the carrier; after the ultrasonic welding, conveying
portions of the metallic foil and the carrier past a second set of
one or more cleaning stations; and at the second set of cleaning
stations, applying cleaning solution to clean the metallic foil and
the carrier.
8. The method of claim 1, further comprising: after the ultrasonic
welding, cutting the foil carrier structure to form a plurality of
slots in the foil carrier structure, each of the plurality of slots
penetrating entirely through the metallic foil and the carrier,
wherein the plurality of slots are arranged to help divide the foil
carrier structure into sections, thereby helping to contain heat
expansion within each section and reduce warpage in the foil
carrier structure.
9. The method of claim 8, wherein a subset of the slots are
arranged in the middle of the panel and extend across at least a
majority of the width of the panel.
10. The method of claim 8, wherein each slot of a subset of the
slots is a notch at an edge of the panel.
11. The method of claim 10, wherein the ultrasonic welding forms a
welding line on the metallic foil that is non-continuous over each
slot but is otherwise continuous, the welding line forming a
rectangle and extending along the periphery of the panel.
12. The method of claim 1, wherein the metallic foil and the
carrier have coefficients of thermal expansion (CTE) at 20.degree.
C. that differ by less than 10.sup.-6/C, thereby helping to reduce
warpage in the metallic foil and the carrier.
13. The method of claim 12, wherein the metallic foil is made of
copper and the carrier is made of aluminum alloy CE17.
14. The method of claim 1, wherein: the ultrasonic welding involves
welding a carrier surface of the carrier to an opposing foil
surface of the metallic foil, the carrier surface and the foil
surface each having a surface area of at least approximately 7500
mm.sup.2; and subjecting the foil carrier structure to a
temperature increase of greater than approximately 150.degree. C.
while limiting the warpage of the foil surface to approximately 5
mm or less without applying any substantial pressure external to
the foil and the carrier on the carrier surface and the foil
surface.
15. The method of claim 14, wherein the thickness of the metallic
foil is between approximately 8 and 35 microns and the thickness of
the carrier is between approximately 7 and 25 mils.
16. A foil carrier structure for packaging integrated circuits,
comprising: a carrier; a metallic foil ultrasonically welded to the
carrier to form a foil carrier structure, the ultrasonic welding
defining a panel in the metallic foil that is suitable for use in
packaging integrated circuits.
17. The foil carrier structure of claim 16, wherein the ultrasonic
welding forms an intermittent welding pattern that extends along
edges of the panel, the intermittent welding pattern including
ultrasonically bonded portions of the metallic foil interspersed
among unbonded portions of the metallic foil, the unbonded portions
being portions of the metallic foil that have not been
ultrasonically welded to the carrier.
18. The foil carrier structure of claim 17 further comprising: a
multiplicity of integrated circuit dice mounted onto the metallic
foil; and a molding material that encapsulates the multiplicity of
integrated circuit dice and at least portions of the metallic
foil.
19. The foil carrier structure of claim 16, wherein the foil
carrier structure includes a plurality of slots, each of the
plurality of slots penetrating entirely through the metallic foil
and the carrier, wherein the plurality of slots are arranged to
help divide the foil carrier structure into sections, thereby
helping to contain heat expansion within each section and reduce
warpage in the foil carrier structure.
20. The foil carrier structure of claim 16, wherein: the metallic
foil includes a foil surface; the carrier including a carrier
surface, the carrier surface being ultrasonically welded to the
foil surface, the carrier surface and the foil surface each having
a surface area of at least 7500 mm.sup.2; and the metallic foil and
the carrier being arranged such that, when the welded foil carrier
panel arrangement is subjected to a temperature increase of at
least 150.degree. C., the warpage of the foil surface and the
carrier surface is limited to 5 mm or less without applying any
substantial pressure external to the foil and the carrier on the
carrier surface and the foil surface.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a Continuation-in-Part of and claims
priority to U.S. patent application Ser. No. 12/133,335, entitled
"Foil Based Semiconductor Package," filed Jun. 4, 2008, which is
hereby incorporated by reference in its entirety for all
purposes.
TECHNICAL FIELD
[0002] The present invention relates generally to the packaging of
integrated circuits. More particularly, the present invention
relates to packaging methods and arrangements involving thin
foils.
BACKGROUND OF THE INVENTION
[0003] There are a number of conventional processes for packaging
integrated circuit (IC) dice. By way of example, many IC packages
utilize a metallic leadframe that has been stamped or etched from a
metal sheet to provide electrical interconnects to external
devices. The die may be electrically connected to the leadframe by
means of bonding wires, solder bumps or other suitable electrical
connections. In general, the die and portions of the leadframe are
encapsulated with a molding material to protect the delicate
electrical components on the active side of the die while leaving
selected portions of the leadframe exposed to facilitate electrical
connections to external devices.
[0004] Many conventional leadframes have a thickness of
approximately 4-8 mils. Further reducing the thickness of the
leadframe offers several benefits, including the potential of
reducing the overall package size and conserving leadframe metal.
In general, however, a thinner leadframe has a greater propensity
to warp during the packaging process. A supporting structure, such
as backing tape, may be applied to the leadframe to reduce the risk
of warpage. Such structures, however, may entail higher costs.
[0005] At various times, package designs have been proposed that
utilize a metal foil as the electrical interconnect structure in
place of the leadframe. Although a number of foil based designs
have been developed, none have achieved widespread acceptance in
the industry in part because foil based packaging processes tend to
be more expensive than conventional leadframe packaging and in part
because much of the existing packaging equipment is not well suited
for use with such foil based package designs.
[0006] Although existing techniques for fabricating leadframes and
for packaging integrated circuits using leadframe technology work
well, there are continuing efforts to develop even more efficient
designs and methods for packaging integrated circuits.
SUMMARY OF THE INVENTION
[0007] In one aspect of the present invention, methods for
minimizing warpage in a thin foil used in integrated circuit
packaging are described. Portions of a metallic foil are
ultrasonically welded to a carrier to form a foil carrier
structure. The ultrasonic welding helps define a panel in the
metallic foil that is suitable for packaging integrated circuits.
One embodiment of the present invention involves forming an
intermittent welding pattern that extends along the edges of the
panel. In another implementation, notches and/or slots are cut in
the foil carrier structure. In still another embodiment of the
present invention, the materials for the metallic foil and the
carrier are selected to have similar coefficients of thermal
expansion. Additionally, the thicknesses of the metallic foil and
the carrier may be selectively correlated to reduce heat-induced
warpage in the foil.
[0008] In another aspect of the present invention, foil carrier
structures for use in the aforementioned methods are described.
BRIEF DESCRIPTION OF THE DRAWINGS
[0009] The invention and the advantages thereof, may best be
understood by reference to the following description taken in
conjunction with the accompanying drawings in which:
[0010] FIG. 1A is a diagrammatic top view of a foil carrier panel
with an intermittent welding pattern according to one embodiment of
the present invention.
[0011] FIG. 1B is a diagrammatic top view of a foil carrier panel
with slots and notches according to one embodiment of the present
invention.
[0012] FIG. 1C is a diagrammatic side view of a foil carrier panel
according to one embodiment of the present invention.
[0013] FIG. 2 is a flow chart illustrating a process for packaging
an integrated circuit device in accordance with one embodiment of
the present invention.
[0014] FIGS. 3A-3E are diagrammatic side views of various stages of
the packaging process in accordance with one embodiment of the
present invention.
[0015] FIG. 4A is a diagrammatic top view of an example etching
carrier after the molded foil structure illustrated in FIG. 3E has
been placed in the carrier.
[0016] FIG. 4B is a diagrammatic top view of the etching carrier
and molded foil structure illustrated in FIG. 4A after etching.
[0017] FIG. 4C is an enlarged diagrammatic top view of a device
area resulting from the etching process of FIG. 4B according to one
embodiment of the present invention.
[0018] FIGS. 5A-5C are diagrammatic side views of the molded foil
structure illustrated in FIG. 3D after etching, electroplating and
singulation.
[0019] FIG. 5D is a diagrammatic side view of a singulated package
according to one embodiment of the present invention.
[0020] FIG. 5E is a diagrammatic bottom view of the singulated
package illustrated in FIG. 5D.
[0021] FIGS. 6A and 6B are diagrammatic side and top views of a
metallic foil and a carrier being unwound from coils according to
one embodiment of the present invention.
[0022] FIG. 6C is a diagrammatic top view of a foil carrier panel
arrangement according to one embodiment of the present
invention.
[0023] In the drawings, like reference numerals are sometimes used
to designate like structural elements. It should also be
appreciated that the depictions in the figures are diagrammatic and
not to scale.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0024] The present invention relates generally to the packaging of
integrated circuits using thin foils. Various approaches for
incorporating thin foils into integrated circuit packaging involve
welding a thin foil to a carrier to form a foil carrier structure.
At various stages in the packaging and assembly process (e.g., die
attach cure, wire bonding, molding, etc.), the foil carrier
structure is subjected to high temperatures. Generally, since the
carrier and the foil are welded together, temperature cycling can
cause frame warpage due to the CTE mismatch between the carrier and
the foil, which may cause problems during package assembly and
degrade the performance and reliability of the resulting integrated
circuit package. Although pressure can be applied to the thin foil
to arrest warpage, this generally requires additional process steps
and/or materials. Accordingly, the present invention pertains to
arrangements and methods for reducing warpage while minimizing or
eliminating the need for applying such pressure to the foil.
[0025] Referring now to FIGS. 1A-1C, exemplary foil carrier panels
for use in IC packaging are described. FIG. 1A illustrates a
diagrammatic top view of a foil carrier panel 100 according to one
embodiment of the present invention. A metallic foil 101 is
ultrasonically bonded to an underlying carrier (not shown) using an
intermittent welding pattern 102. The intermittent welding pattern
102 intersperses bonded portions 103 with unbonded portions
105.
[0026] Generally, if the metallic foil 101 and the underlying
carrier have substantially different coefficients of thermal
expansion (CTE), they will expand at different rates when subjected
to an increase in temperature. The difference in the rates of
expansion can cause tension at bonded portions 103. The
intermittent welding pattern 102, however, provides stress relief
by allowing expansion at the unbonded portions 105. As a result,
the overall warpage of the metallic foil is reduced.
[0027] The intermittent welding pattern 102 may be arranged in any
appropriate manner, as long as each bonded portion 103 is adjacent
to and/or surrounded by the unbonded portions 102. By way of
example, bonded portions 103 with a pitch of between approximately
10 and 20 mm works well in various applications, although larger
and smaller pitches are also possible. (Pitch can be understood as
the distance between the centers of adjacent pairs of bonded
portions 103.) In some embodiments, the length of the unbonded
portion 105 that separates two adjacent bonded portions 103 may be
approximately between 10 and 20 mm and the length of each bonded
portion 103 may be between 3 and 7 mm. Preferably, multiple bonded
portions 103 are arranged with a substantially uniform pitch in
lines along all four edges of the rectangular foil carrier panel
100. Such an arrangement helps secure all 4 sides of the foil
carrier panel 100 and helps distribute tension uniformly around the
periphery of the panel.
[0028] Referring next to FIG. 1B, a foil carrier panel 102 in
accordance with another embodiment of the present invention will be
described. Slots 108 and notches 106 have been cut in the foil
carrier panel 102, which includes a metallic foil 109 that has been
ultrasonically welded to an underlying carrier (not shown) using
continuous welding lines 104. In the illustrated embodiment, the
slots 108 and notches 106 penetrate entirely through the foil 109
and the underlying carrier, although this is not a requirement.
[0029] The formation of slots 108 and notches 106 in the foil
carrier panel 102 are another means of reducing the warpage of the
foil carrier panel 102. Generally, when an uncut foil carrier panel
is subjected to high temperatures, expansion occurs along the
entire length of the panel. In the illustrated embodiment, the
slots 108, which extend across the majority of the width of the
foil carrier panel 102 and are arranged in the middle of the panel,
effectively divide the foil carrier panel 102 into sections 111 and
helps limit expansion to each section. The notches 106, which
extend into the foil carrier panel 102 from its edges, provide
stress relief by breaking up the welding lines 104.
[0030] Warpage reduction can also be achieved by adjusting the
thickness of the thin foil relative to its underlying carrier. This
approach will be discussed with reference to FIG. 1C, which is a
diagrammatic side view of a foil carrier panel 110. Foil carrier
110 includes a thin foil 112 with a thickness 116 that is
ultrasonically bonded to a carrier 114 with a thickness 118.
Generally, the exterior surface of thinner layers, when subjected
to an increase in temperature, expand faster than the exterior
surface of thicker layers. By taking into account the CTEs of the
foil 112 and carrier 114 and adjusting their thicknesses 116 and
118 accordingly, the mismatch in the rate of expansion between the
foil surface 120 and the carrier surface 122 can be reduced. As a
result, the foil surface 120 is not pulled and warped as much by
its bonding to the carrier surface 122.
[0031] Various tests have been performed to help confirm the
efficacy of the aforementioned approaches. In one experiment, a
foil carrier panel was used that was formed by ultrasonically
welding a copper foil to an aluminum carrier with a single,
continuous welding line that extended along the periphery of the
panel. The foil carrier panel had dimensions of approximately
165.times.65 mm. The copper foil had a thickness of approximately
18 microns. The aluminum carrier had a thickness of approximately 7
mils. The foil carrier panel was subjected to a temperature
increase from room temperature to approximately 175.degree. C. The
resulting warpage of the foil carrier panel was approximately 30
mm. (For the purposes of this experiment, the warpage of the foil
carrier panel is understood as the maximum linear displacement of
the foil carrier panel as a result of the temperature increase, as
measured along an axis that is perpendicular to the foil and
carrier surfaces.) The same test conditions were repeated in a
second experiment, except that the thickness of the aluminum
carrier was changed to approximately 20 mm and the foil was
ultrasonically welded to the carrier using a stitched, intermittent
bonding pattern. The warpage of the foil carrier panel in the
second experiment was approximately 3 mm, which constitutes a 10
fold decrease in warpage. More generally, when a foil carrier
structure having appropriately calibrated foil and carrier
thicknesses and a surface area of at least 7500 mm.sup.2 is
subjected to a temperature increase in excess of approximately
150.degree. C., the warpage of the foil carrier structure may be
limited to approximately 5 mm or less. Aside from the pressure
exerted upon the foil by the ultrasonic bonding, this result can be
achieved without applying substantial additional pressure on the
foil and/or carrier surface (e.g., without applying a tape to the
foil, without having the semiconductor processing equipment apply
pressure on the foil surface to suppress warpage, etc.)
[0032] Warpage can also be addressed by selecting materials for the
foil 112 and the carrier 114 that are suitable for integrated
circuit packaging and that have similar CTEs. By way of example, a
foil and carrier that have CTEs at 20.degree. C. that differ by
less than 10.sup.-6/C work well for various applications.
Accordingly, a suitable pairing would be a foil 112 made of copper
and a carrier 114 made of Aluminum CE17. The two materials are
suitable for use in foil-based integrated circuit packages and both
have CTEs of approximately 18.
[0033] It should be appreciated that any of the various approaches
discussed above in connection with FIGS. 1A-1C can be combined or
modified, depending on the needs of a particular application. By
way of example, another embodiment of the present invention is a
foil carrier panel with both an intermittent welding pattern (as
shown in FIG. 1A) and slots (as shown in FIG. 1B). Various
implementations involve intermittent welding patterns, slots and
notches that differ in quantity, orientation, size and/or shape
from those depicted in FIGS. 1A-1C.
[0034] Referring next to FIGS. 2-5, a method 200 of forming
integrated circuit packages using the thin foil arrangements
illustrated in FIG. 1 will be described. Initially, in step 202, a
foil 306 and a carrier 308 of FIG. 3A is provided. In some
embodiments, the foil 306 is made of copper and the carrier 308 is
made of aluminum, although the foil 306 and the carrier 308 can be
made of other suitable materials as well. For example, the foil 306
can include multiple layers and/or metals, such as copper, nickel,
and palladium. Carrier 308 can be made of any suitable material
e.g., stainless steel, steel, plastic, FR4, etc. Various
implementations use a foil 306 having a thickness between 8 and 35
microns and/or a carrier having a thickness between 7 and 25
mils.
[0035] Afterward, the foil 306 is ultrasonically bonded with the
carrier 308 to form a foil carrier structure 300 (step 203 of FIG.
2). The ultrasonic bonding may be performed to form any of the
welding arrangements discussed above in connection with FIG. 1
(e.g., continuous welding lines, intermittent welding patterns,
etc.) Ultrasonic bonding offers the benefit of being strong enough
to endure stresses imposed by later stages of the packaging process
while still allowing the carrier to be easily separated from the
foil after dice and molding material have been added to the foil.
The term ultrasonic bonding, as used herein, includes any suitable
bonding technique having an ultrasonic component, including
thermosonic bonding. Although ultrasonic bonding works well, it
should be appreciated that other suitable bonding techniques may be
used to secure the foil to the carrier. By way of example, a
variety of suitable adhesives or tape may be used. Various
approaches to ultrasonically welding and forming the foil carrier
structure 300 are discussed in U.S. patent application Ser. No.
12/133,335, entitled "Foil Based Semiconductor Package," filed Jun.
4, 2008, which is hereby incorporated by reference in its entirety
for all purposes.
[0036] Preferably after ultrasonic bonding, the foil carrier
structure 300 may be optionally cut to form one or more of the
slots and/or notches discussed above in connection with FIG. 1B
(step 203 of FIG. 2). Generally, the ultrasonic bonding helps
maintain the alignment of the foil 306 and the carrier 308 during
the cutting operation. In some embodiments, the cutting operation
may also take place before and/or substantially at the same time as
the ultrasonic bonding.
[0037] As a result of the aforementioned ultrasonic bonding and/or
cutting operations, one or more foil carrier panels of FIGS. 1A, 1B
and/or 1C are formed in the foil carrier structure 300 of FIG. 3A.
The foil carrier structure 300 may be arranged in any suitable
manner. By way of example, the foil carrier structure 300 may be
formed from strips that are unwound from coils and subsequently
welded together. In still another embodiment, the foil carrier
structure 300 is a rectangular panel arrangement that includes
multiple foil carrier panels. (These implementations are discussed
later in connection with FIGS. 6A-6C.)
[0038] Referring now to step 204 of FIG. 2 and FIG. 3B, dice 318
are attached to the foil carrier structure 300 using conventional
die attach techniques. In the illustrated embodiment, the dice 318
are attached to the foil 306 using conventional die attach material
and further attached to the foil 306 with conventional wire bonds
316, although any suitable electrical connection (e.g., solder
bumps in a flip chip-style arrangement, etc.) may be used. Although
FIG. 3B shows the foil carrier structure supporting only several
dice 318, it should be appreciated that the foil carrier structure
300 can be sized and arranged to support hundreds of dice or
more.
[0039] In step 206 and FIG. 3C, dice 318 and at least a portion of
the top surface of the foil 306 are encapsulated with a molding
material 322, forming molded foil carrier structure 324. In the
illustrated embodiment of FIG. 3C, molding material 322 is added in
a single continuous strip. That is, the molding material has been
relatively evenly applied across the molded portions of foil 306.
It is noted that this type of molding is not common in leadframe
based packaging. Rather, the devices carried on leadframe strips
are typically molded either individually or in sub-panels. The
benefits of a continuous strip of molding material will be
discussed in connection with FIGS. 3D, 3E and step 208.
[0040] In step 208, the carrier portion of molded foil carrier
structure 324 of FIG. 3C is removed, resulting in molded foil
structure 325 of FIG. 3D. At this point the molding material 322
provides structural support for the foil in place of the carrier
308. It should be appreciated that an advantage of the continuous
strip molding approach is that it provides good support for the
entire panel so that the strip may still be handled in panel form.
In contrast, if molding gaps are provided between subpanels during
the molding operation, then the subpanels would need to be handled
independently after removal of the carrier.
[0041] FIG. 3E presents an external view of molded foil structure
325. It should be appreciated that although the top surface 328 of
molded foil structure 325 is substantially planar, this is not a
requirement. Molding material 322 in molded foil structure 325 may
assume a variety of patterns and shapes, and the depth 334 of
molding material 322 may vary along the length of molded foil
structure 325.
[0042] Referring to step 213 of FIG. 2, the exposed foil 306 of the
molded foil structure 325 of FIG. 3E is then patterned using known
photolithographic techniques. In various embodiments, a photoresist
layer is applied over the foil 306. Portions of the photoresist
layer are selectively exposed to light. A developer solution is
then applied to remove portions of the photoresist layer to form a
desired pattern. A wide variety of approaches known to persons of
ordinary skill in the art can be used to pattern the exposed foil
306.
[0043] In optional step 209, molded foil structure 325 is placed in
etching carrier 404 as illustrated in FIGS. 4A and 4B. It should be
noted that the use of the etching carrier 404 is not required and
the molded foil structure 325 can be etched using any suitable
device or process e.g., an etching conveyor, etc. FIG. 4A
illustrates a top view of etching carrier 404 containing molded
foil structure 325. In the illustrated embodiment, etching carrier
404 includes alignment holes 402 and a cavity 406 configured to
receive molded foil structure 325. Etching carrier 404 is designed
to receive molded foil structure 325 of FIG. 3E such that the top
surface 328 of the molded foil structure is hidden within cavity
406 and foil 306 is exposed. The etching carrier may be reusable
and can be made of various materials, such as fiber glass.
[0044] In step 210, foil 306 is etched using any suitable technique
known to persons of ordinary skill in the art, such as chemical
etching. As shown in FIGS. 4B, 5A and 5B, the etching removes
portions of foil 306 and defines multiple device areas 410. Each
device area 410 is arranged to support one or more of the dice 318
of FIG. 3.
[0045] Some embodiments involve forming device areas 410 with bus
bars in order to facilitate the later electroplating of a metal,
such as tin or solder, on electrical contacts formed from the foil.
FIG. 4C diagrammatically illustrates such a device area. In the
illustrated embodiment, device area 410 has a die attach pad 412,
contact leads 414 and bus bars 416. Bus bars 416 electrically
connect the pad and leads. Bus bars 416 may also form conductive
links between multiple device areas. It should be appreciated that
device area 410 represents only one of many possible
arrangements.
[0046] FIGS. 5A-5B provide a diagrammatic side view of the effect
of the etching process on molded foil structure 325. FIG. 5A is a
diagrammatic side view of molded foil structure 325 prior to
etching. FIG. 5B illustrates how the etching process removes
portions of foil 306, reveals sections of molding material 322 and
forms contact leads 414 and die attach pad 412.
[0047] As discussed above, some embodiments contemplate step 211 of
FIG. 2, which involves the electroplating of solder 508 of FIG. 5C
onto die attach pad 412 and contact leads 414. In step 212, the
molded foil structure 325 is singulated along projected saw streets
302 of FIG. 5C to form semiconductor packages. Molded foil
structure 325 may be singulated using a variety of techniques,
including sawing and laser cutting. An enlarged side view of
singulated package 520 is illustrated in FIG. 5D. A diagrammatic
bottom view of the package is shown in FIG. 5E. The bottom view
illustrates die attach pad 516 and contact leads 518 surrounded by
molding material 322.
[0048] The processes described above in connection with FIGS. 3-5
can be modified as appropriate for particular applications. By way
of example, application Ser. No. 12/571,202, entitled "Foil Based
Semiconductor Package," filed Sep. 30, 2009 by the assignee of the
present application, which is hereby incorporated by reference in
its entirety for all purposes, describes various die attach,
etching and processing steps for thin foils. More particularly, the
'202 application generally relates to attaching dice to a thin foil
in a flip chip arrangement, etching the thin foil to form device
areas, and selectively applying a dielectric material over portions
of each device area. Any of the operations described in the '202
application can be combined with and/or replace the operations
described above.
[0049] Referring next to FIGS. 6A and 6B, additional arrangements
and methods for forming the foil carrier structure 300 of FIG. 3A
will be described. FIGS. 6A and 6B illustrate diagrammatic side and
top views of a foil coil 600 and a carrier coil 601 according to
one embodiment of the present invention. As seen in FIG. 6A, a
metallic foil 306 is unwound from the foil coil 600 while the
carrier 308 is unwound from the carrier coil 601. Initially,
unwound sections of the foil 306 and the carrier 308 may be cleaned
by conveying them past one or more optional cleaning stations (not
shown), which apply cleaning solutions to the foil and the carrier.
A welding device 606 (e.g., an ultrasonic horn, etc.) then
ultrasonically welds the superimposed foil 306 and carrier 308 to
form any of the welding patterns discussed in connection with FIGS.
1A-1C. Afterward, the welded sections may be passed by additional
cleaning stations and/or subjected to a first set of cutting
operations, which may form any of the slots and/or notches
discussed in connection with FIG. 1B. As seen in FIG. 6B, the
welding and/or cutting operations form one or more foil carrier
panels 608, which each have various features discussed in
connection with foil carrier panels 100, 102 and/or 110 of FIG. 1.
Preferably, the above welding and cutting operations take place
while sections of the foil 306 and the carrier 308 are in motion
and being unwound from the foil coil 600 and the carrier coil 601
(although this is not a requirement). Such an approach can help
streamline package assembly and reduce the number of processing
steps. A second set of optional cutting operations may then be
performed to form appropriately sized foil carrier structures.
Afterward, the die attach, encapsulation, etching and/or
singulation steps described in connection with FIGS. 3B-3E, 4 and 5
can be applied to the resulting foil carrier structure(s).
[0050] Referring now to FIG. 6C, another approach for forming the
foil carrier structure 300 of FIG. 3A is described. FIG. 6C is a
diagrammatic top view of a foil carrier panel arrangement 616
according to another embodiment of the present invention. A
metallic foil 306 is ultrasonically bonded to an underlying carrier
(not shown). The ultrasonic bonding defines multiple foil carrier
panels 610. Welding and/or cutting operations can be performed on
the foil carrier structure 300 to form multiple panels 610 that
each have intermittent welding patterns, slots, notches and/or any
other feature described in connection with the panels illustrated
in FIG. 1. Afterward, the foil carrier structure 300 may be
optionally cut along projected saw streets 614 as appropriate. Die
attach, encapsulation, etching and/or singulation steps described
in connection with FIGS. 3B-3E, 4 and 5 can each be applied on the
level of an individual panel 610 or on the level of an entire panel
arrangement 616.
[0051] Although only a few embodiments of the invention have been
described in detail, it should be appreciated that the invention
may be implemented in many other forms without departing from the
spirit or scope of the invention. By way of example, FIG. 1B
depicts two slots 108 of FIG. 1B, which each extend across a
majority of the width of the panel 102. The present invention also
contemplates fewer or more slots per panel and slots that each have
different dimensions relative to the width of the panel. Although
FIG. 1B illustrates only 4 notches that are positioned near the 4
ends of the slots 108, there may also be fewer or more notches in a
different arrangement. FIG. 1A has bonded portions 102 that are
linearly arranged and rectangular. The long sides of all of the
rectangular bonded portions 102 run parallel to the long sides of
the rectangular panel 100. In various embodiments, the bonded
portions 102 may be oriented vertically rather than horizontally,
have different shapes and/or be arranged in a different layout. It
should be further appreciated that the steps of FIG. 2, while
illustrated in a specific order, may be reordered as appropriate.
Additionally, one or more steps may be replaced and/or eliminated
for particular applications. Therefore, the present embodiments
should be considered as illustrative and not restrictive and the
invention is not limited to the details given herein, but may be
modified within the scope and equivalents of the appended
claims.
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